Steele, who oversees the $200 million effort funded in part by the Bill & Melinda Gates Foundation, said his last day will be July 11.

"My house is paid off and I can reserve more time for my family," he said. "I absolutely plan to relax."

Steele said he does not expect the project, which has given the district national recognition, to suffer.

Neither does superintendent MaryEllen Elia, who praised Steele as "a tremendous resource for the district, in many different positions."

Part of his legacy, she said, is that the teacher improvement program called Empowering Effective Teachers, or EET, is well positioned to continue under a new leader.

A 1973 graduate of Plant City High School, Steele was a math teacher, an assistant principal and a principal. During his 35-year career he held district-level positions in technology and instruction.

His official title is chief information and technology officer.

But he's held the top job in EET almost since the district won a seven-year, $100 million grant from the Gates Foundation to cover half the funding. The district has matched that with $100 million of its own.

The program, launched more than a year before the state mandated its own reforms, seeks to support new teachers with training and mentoring, provide comprehensive feedback for experienced teachers and reward teachers based on performance instead of seniority.

It has had mixed reaction from teachers, who generally appreciate the mentoring component but not necessarily the evaluations.

Teachers are observed separately by administrators and peers under a highly structured format. And, as in the rest of the state, there is a "value-added" component of their score that measures student growth based on tests.

Hillsborough prides itself on having an unusually sophisticated system. Student data is adjusted for variables such as absenteeism and learning disabilities. Performance data comes from the teachers' own students, as opposed to school-wide averages used elsewhere.

"You are judged on your students and your subject," Steele said Wednesday. "Virtually no one else in the nation can say that."

Steele has been the program's ambassador in what could have been a tumultuous transition for some 15,000 teachers. Much of his job entailed explaining the process to faculty and community groups.

He acknowledged in 2011 that it would be hard for most teachers to understand the many calculations that go into their value-added scores.

"You have this line between transparency and accuracy," he said. But accuracy requires such complex formulas that for a teacher to understand them, "you would need a Ph.D. in mathematics."

Addressing the issue of teacher firings in a later interview, he said, "My goal would be to have zero dismissals, like any school system's would be, but to do it genuinely through improvement of practice."

The ambitious nature of EET and the unusual degree of collaboration between the district and its teachers union — which predates the Gates grant — boosted the profiles of both Elia and union president Jean Clements.

Clements said she was "crushed" when Steele confirmed he was leaving.

Having served on numerous panels with Steele, she found him refreshingly candid about the challenges Hillsborough faces. She's seen him handle pointed questions and criticism from groups of teachers without becoming defensive.

"I have never seen him angry or impatient or ruffled in any way," she said.

Elia agreed that Steele's temperament has been one of his assets.

"I think he is a listener and he is very data driven," she said.

Steele acknowledged, "patience is a strong suit that has served me well always." He said the district should be patient too.

"Staying the course is very important," he said. "Each year we will make tweaks at the district level. And as we work through time, more and more people will accept it as our way of work."

April Griffin, chairwoman of the School Board, said she is not worried about a succession plan.

"I think that we have documented EET so well that we do not need to be concerned about Dr. Steele's absence in that regard," she said.

Steele said he's confident his staff is capable of continuing the program.

"All of the directors have three years experience and have done every bit as much as I have shaping the program," he said. "I'm surprised that would even be a question."